Pneumatic tube conveyors are in common use in stores, apartment buildings, hospitals, office buildings and the like for carrying messages and supplies and delivering mail between various points in the buildings. Such systems operate by generating a pressure differential between two points, either by pressurizing at the sending point or producing a vacuum at the receiving point. The carriers have to fit relatively snugly within the tube so that they can function as free pistons to be projected by the pressure differential across them. Because the carriers fit fairly snugly, changes in direction require bends of extremely large radius, which often require that curved sections of tubing extend into space which could be utilized otherwise.
Conventionally, pneumatic tube conveyors are circular in cross-section to facilitate sealing around the carriers. However, this requires that envelopes, photographs and the like of larger dimension than the diameter of the carrier be rolled or curled in order to fit within the carrier. It would be advantageous to have a rectangular shaped carrier to travel through a pneumatic tube of like configuration, but the difficulty lies in the condition encountered when the carrier is made to travel through tubing bends, as described above.